In the past, various manners of storing and providing electrical energy to drive an electrical load, such as an electrical driving motor, have been proposed. For example, different types of batteries, including lead-acid, nickel cadmium (Ni—Cd) and nickel metal hydride (Ni—MH), have been used in the past to drive electric vehicles. However, each type of battery has unique advantages and disadvantages.
For example, lead-acid batteries have the advantage that they can provide a high burst of power when required. Moreover, lead-acid batteries can provide large currents sufficient to accelerate and drive electrical loads, such as electrical motors and engines in vehicles. However, lead-acid batteries suffer from the disadvantage of having low energy density, sometimes expressed or measured, as Watt-hour per liter (W-h/l), meaning that the energy provided per unit volume is low. Likewise, lead-acid batteries have relatively low specific energy, expressed as watt-hour per kilogram (W-h/kg), meaning that a relatively large mass is needed to store a substantial quantity of energy.
By contrast, lithium-based batteries, such as lithium batteries having anodes or negative electrodes of lithium metal or alloy, and non-aqueous rechargeable lithium ion batteries, as disclosed for instance in U.S. Pat. No. 6,159,635, issued to Das Gupta et al., have higher energy density and specific energy characteristics than lead or nickel based electrochemical cells. It should be noted, that some types of non-aqueous rechargeable lithium ion batteries are referred to as polymer lithium batteries, due to being packaged and sealed in polymer layers and having lithium ion conducting polymer electrolytes. On the other hand, lithium based batteries may not be able to provide large bursts of power, in particular, high current densities, on account of the intrinsic high impedance of such lithium based cells. Furthermore, to prevent degradation, lithium based cells require thermal management techniques to maintain the battery at an acceptable temperature, such as −20° C. to a maximum of 70° C. Power bursts in lithium ion cells generally generate larger amounts of heat energy, which, if not managed properly, can degrade the battery.
In an electrical vehicle, it is desirable to have an energy storage device which has a high energy density, so that a minimum volume is occupied by the energy storage device, as well as a high specific energy, so that minimum weight is transported along with the vehicle. However, it is also desirable to have an energy storage device which can provide large bursts of power. In particular, a burst of power is generally required to overcome stationary friction and the inertia of a stationary electrically driven vehicle, as well as for acceleration. It is noted that attempts have been made to redesign rechargeable lithium batteries to be able to provide higher currents, but this led to lower specific energies and lower energy densities of such battery devices.
In the past, several different types of energy storage devices have been proposed in an effort to provide a high energy storage device that provide large bursts of power. For example, U.S. Pat. No. 5,780,980 and U.S. Pat. No. 5,808,448, both to Naito, disclose an electric car drive system having a direct current power supply comprising a fuel cell connected to a lead-acid battery. The fuel cell produces a constant output while operational and supplies electrical power to the car when the power rate for the electrical load is low. When the power rate for the electrical load increases, power is supplied by the lead-acid battery, as well as by the fuel cell. Naito also discloses that the fuel cell recharges the lead-acid battery when the charge for the lead-acid battery is below a specified value. However, Naito suffers from the disadvantage that the fluid reactants to operate the fuel cell must be carried in containers on the vehicle. This greatly reduces the specific energy capability of the device. Also, Naito discloses an elaborate electrical circuit to permit supply of energy from the fuel cell and the lead-acid battery.
European Patent Office application number 0 564 149 A2 to Okamura, discloses utilizing capacitors connected in series and in parallel, but does not disclose the use of batteries. Furthermore, Okamura discloses specific circuits for detecting whether or not the capacitor is at the fully charged level to prevent over charging. Likewise, European Patent Office application number 0 410 559 A2 to Shirata discloses using capacitors, but Shirata also relates to using the capacitors to energize a starter motor which in turn starts a gasoline engine. Similarly, U.S. Pat. No. 5,998,960 to Yamada discloses using a capacitor with a battery in combination with a gasoline engine to assist in regenerative braking, and, other means to limit the use of the gasoline engine and thereby limit fuel consumption and reduce exhaust gases. In this way, both Yamada and Shirata are not directly concerned with storing large amounts of power, because both disclose use of the power storage system in combination with gasoline or other fossil fuel engines. Furthermore, both Yamada and Shirata relate to circuits which are focused on their specific purposes; for Shirata this relates to assisting the starter engine to start the engine, and, for Yamada, this relates to using a chopper to maintain the voltage, such as during regenerative braking, at specific levels.
For much smaller loads, such as in the micro-electronic field, as used in electrochromic eye wear, lithium/thionylchloride and lead-acid hybrid batteries have been proposed. For instance, U.S. Pat. Nos. 5,900,720 and 5,455,637 to Kallman disclose using a hybrid battery comprising a primary, that is non-rechargeable, lithium/thionyl chloride battery cell and a secondary sealed lead-acid battery to power micro-electronic circuits. The primary and secondary batteries power a load, which in the case of Kallman are low power micro-electronic circuits for electrochromic eye wear. The primary battery also powers a controller which, in turn, can periodically charge the secondary battery. However, Kallman does not disclose that the primary lithium/thionylchloride battery is recharged. Also, the Kallman device is designed to be small with relatively low total energy output, and as such, could not be utilized for larger loads.
Also, capacitors have been used in the past as disclosed, for instance, in European application 0 564 149 A2 to Jeol Ltd. However, as disclosed in this application, capacitors are much more sensitive to the applied voltage and, if the voltage applied to the capacitor exceeds the rated voltage, then the capacitance of the capacitor is immediately reduced and the leakage current increases. Because of this, European application 0 564 149 A2 discloses at length control circuits to limit charging of the capacitors, but has no disclosure relating to use of batteries nor how to control the batteries.
Accordingly, there is a need in the art for an efficient energy storage device having a relatively high energy density and relatively high specific energy for use with large loads having variable power demands. Moreover, while energy density is an important consideration, it is also necessary to consider how the batteries will be housed within the vehicle. In other words, the effective volume of the device including the batteries, meaning the total volume required to house the batteries rather than the volume of the individual cells, must be considered. Yet another consideration should be the charging of the system after the output has dropped below a predetermined level.